1. Field of the Invention
The present invention relates to a method of producing expanded thermoplastic microspheres, more particularly a method of producing expanded thermoplastic microspheres without agglomerate formation, and to an apparatus for expanding said microspheres.
2. Description of the Prior Art
The production and use of expandable thermoplastic microspheres is disclosed in, inter alia, U.S. Patent Specification 3,615,972. The thermosplastic shells of the spheres may consist of, for example, polymers or copolymers that have been polymerised from monomers such as vinyl chloride, vinylidene chloride, acrylonitrile, methyl methacrylate, or styrene, or mixtures thereof. The particle size of the unexpanded spheres, and thus of the expanded spheres, may vary within wide limits and is selected on the basis of the properties desired in the finished product. As examples of particles sizes of unexpanded spheres, mention may be made of 1 .mu.m to 1 mm, preferably 2 .mu.m to 0.5 mm, and especially 5 .mu.m to 50 .mu.m. Upon expansion, the diameter of the microspheres is increased by a factor 2 to 5. The unexpanded spheres contain volatile liquid blowing agents which are gasified upon supply of heat. When heat is supplied, the polymer shell softens and the spheres expand when the blowing agent is gasified. The blowing agents may consist of freons such as trichlorofluoromethane, hydrocarbons such as n-pentane, i-pentane, neopentane, butane, i-butane or other blowing agents conventionally employed in microspheres of the type here referred to. The blowing agent preferably constitutes 5-30% by weight of the weight of the microsphere. One example of a suitable and commercially available microsphere product is Expancel.RTM. which has a thermoplastic shell of a vinylidene chloride/acrylonitrile copolymer and isobutane as blowing agent.
The expandable thermoplastic microspheres of U.S. Pat. No. 3,615,972 are produced in an aqueous suspension. The process water is removed by, for example, sedimentation, centrifugation or filtration, or by some other suitable technique. The microspheres are obtained in the form of a wet cake having a dry solids content of about 65%. The unexpanded microspheres may be used as such for in situ expansion, i.e. the spheres are mixed with different types of material which are then heated to make the microspheres expand. Examples of commercial applications of this type are paper, cardboard and printing ink.
Thermoplastic microspheres also have applications where the temperature is not increased during the process, or the temperature is not sufficiently high to make the microspheres expand. For these applications, use is made of preexpanded microspheres which may be dry or wet. Examples of commercial applications for this type of microspheres is polyester for dry spheres and paint for wet spheres.
Processes for the production of both wet and dry expanded microspheres are known. U.S. Pat. No. 4,397,799 discloses a process of drying and expanding microspheres by atomising a dispersion of the spheres in an inert liquid in a hot inert gas. EP 112807 discloses a process for expanding expandable microspheres, in which a slurry of the unexpanded spheres in an inert liquid is supplied into a pressure zone and expanded by coming into contact with water vapour. The spheres are then caused to leave the pressure zone under a considerable pressure drop. Important factors of these processes are the design of the expansion equipment, the process conditions, such as temperature, time, and the concentration of spheres within the slurry. Without suitable adaptation of these factors, an expanded product is obtained which contains large amounts of agglomerates, i.e. cemented expanded spheres. A minor amount of agglomerates (&lt;1%) cannot be avoided because the microspheres stick to the wall of the expansion equipment and subsequently come loose, resulting in small agglomerates. A minor amount of agglomerates may be tolerated, but is undesirable.
The agglomerate formation has been deemed to depend entirely upon the thermoplastic character of the microspheres. If expansion occurs at a temperature higher than normal, a larger number of agglomerates are obtained. An increase by one or two degrees is sufficient to cause difficulties. Expansion of a microsphere grade having a higher glass transition temperature requires a higher expansion temperature, which results in a larger number of agglomerates or a higher tendency to agglomeration.